feat: create useLocalStorage custom hook and migrate useTheme

[Demilade10]

Summary

Closes #616

Implements a type-safe, generic useLocalStorage<T> custom hook to replace direct localStorage access, and migrates useTheme.ts to use it.

Changes

frontend/src/hooks/useLocalStorage.ts (new)

• Generic useLocalStorage<T>(key, initialValue) returning [storedValue, setValue, removeValue] as const
• setValue accepts a direct value or an updater function (prev: T) => T
• removeValue clears the key from localStorage and resets state to initialValue (never null)
• All reads/writes in try/catch - corrupted/invalid JSON falls back to initialValue silently
• SSR guard: typeof window === 'undefined' check before any window.localStorage access

frontend/src/hooks/useLocalStorage.test.ts (new)

10 tests covering:

• Returns initialValue when key is absent
• Reads an existing value from storage on mount
• setValue with a direct value and with an updater function
• Object serialisation round-trip
• removeValue clears storage and resets state to initialValue (not null)
• Graceful recovery from corrupted JSON (no throw, returns initialValue)

frontend/src/hooks/useTheme.ts (modified)

Replaced direct localStorage.getItem / localStorage.setItem calls with useLocalStorage. External API (useTheme, ThemeProvider, toggleTheme) is unchanged.

frontend/src/hooks/index.ts (modified)

Added: export { useLocalStorage } from './useLocalStorage'

Testing

pnpm exec vitest run src/hooks/useLocalStorage.test.ts 10/10 tests passed

All four touched files pass ESLint with zero errors or warnings.

Screenshots

Hooks-only change - no UI output.

[vercel]

@Demilade10 is attempting to deploy a commit to the dinahmaccodes' projects Team on Vercel.

A member of the Team first needs to authorize it.